Stainless steel pipe with excellent corrosion resistance and manufacturing method thereof

ABSTRACT

A stainless steel pipe may be provided which exhibits excellent corrosion resistance so as not to rust at an early stage even in a waterfront environment affected by sea salt particles. The stainless steel pipe may have a polishing mark on a surface, an oxide film exhibiting color is not present on the surface, and an average number of surface defects including covering by a metal base of 5 μm or more on the surface is suppressed to 5 or fewer per 0.01 mm 2 .

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. national stage of application No. PCT/JP2016/076142,filed on Sep. 6, 2016. Priority under 35 U.S.C. § 119(a) and 35 U.S.C. §365(b) is claimed from Japanese Application No. 2015-197977, filed onOct. 5, 2015, the disclosure of which is also incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a stainless steel pipe exhibitingexcellent corrosion resistance and a method of manufacturing the same.

BACKGROUND ART

Stainless steel is widely used in building material applications such asroofing materials, wall materials, and building components since itexhibits excellent weather resistance, processability, weldability, andthe like. In addition, a stainless steel pipe is used in applicationssuch as handrails, fences, and pipe shutters after being subjected tosurface polishing since it exhibits excellent designing property aswell.

In the general industrial polishing of this stainless steel pipe,scratch-removing polishing is first conducted in order to removescratches and the like on the original pipe before being polished andthen finish polishing, glossy polishing, and the like are conducted. Drypolishing using a flap wheel, a polishing belt, or the like is conductedin the rough polishing and finish polishing in this polishing operation.Furthermore, there is a case in which wet polishing by buffing isconducted after the above process in order to obtain a desired surface.

Conventionally, stainless steel exhibits excellent weather resistance asa material, but there is a case in which the inherent weather resistanceof the material is not exerted and remarkable rusting occurs dependingon the state of polishing finish, and this is one of the factors toeliminate the stability (reliability) of the weather resistance ofstainless steel. For example, there is a case in which rusting occurs ina short period of about one month after stainless steel is constructedinto outdoor handrails and the like.

It is considered that the oxide film and polishing marks remaining onthe surface of the stainless steel pipe after being polished are thestarting points of rusting. The remaining oxide film is a film formeddue to heat generation at the time of polishing and a Cr-depleted layeris formed right under the oxide film. Hence, rusting proceeds from theoxide film and the Cr-depleted layer right under the oxide film andcorrosion resistance is likely to deteriorate when the oxide filmremains. In addition, with regard to the polishing marks which arescratches engraved on the surface of the stainless steel pipe bypolishing as well, the possibility that it is difficult to remove theoxide film formed by polishing using a flap wheel or the like by buffingand the oxide film remains is higher as the concave portion of thepolishing marks is deeper, and rusting proceeds and corrosion resistanceis likely to deteriorate since the concave portion of the polishingmarks becomes a starting point of rusting.

Patent Document 1 proposes a stainless steel pipe capable of maintainingglossiness and weather resistance for a long period by polishing thesurface to be in a state in which rusting does not occur in a shortperiod even in an outdoor environment.

Patent Document 1: Japanese Unexamined Patent Application, PublicationNo. 2003-56755

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The invention described in Patent Document 1 is a stainless steel pipehaving a surface roughness after the final polishing of Ry 0.6 μm orless and an area ratio of the remaining oxide film of 7.0% or less. Inother words, it is intended to decrease the oxide films remaining in theconcave portions of the polishing marks by setting the surface roughnessafter final polishing to Ry 0.6 μm or less. In addition, it is intendedto suppress progress of rusting starting from the oxide film and theCr-depleted layer right under the oxide film and deterioration incorrosion resistance by setting the area ratio of the remaining oxidefilm to 7.0% or less.

Here, referring to Examples of Patent Document 1, the area ratio of theremaining oxide film is from 3.1 to 6.8% and the oxide film thus remainsin a weather resistance-passed product. Hence, there is still a problemthat rusting may proceed from the remaining oxide film and theCr-depleted layer right under the oxide film and the corrosionresistance may deteriorate.

Furthermore, the demand for construction has increased in associationwith the redevelopment of urban areas and the demand for construction inthe waterfront environment has increased in recent years. In thewaterfront environment, there is a problem that building components arelikely to be affected by sea salt particles which are a kind of aerosolparticles contained in the atmosphere and are fine particles composed ofsalts derived from seawater. Hence, needs for highly corrosion resistantbuilding components have increased.

In Patent Document 1, SUS304 is mentioned as one steel type of stainlesssteel pipes exhibiting excellent weather resistance. However, SUS304 hasa problem that it rusts at an early stage and requires maintenance in awaterfront environment affected by sea salt particles.

The present invention has been made to solve the problem described aboveand an object thereof is to provide a stainless steel pipe whichexhibits excellent corrosion resistance so as not to rust at an earlystage even in a waterfront environment affected by sea salt particlesand a method of manufacturing the same.

Means for Solving the Problems

The present inventors have carried out investigations on the stainlesssteel pipe described in Patent Document 1. In Examples of PatentDocument 1, dry polishing using a flap wheel is conducted. The oxidefilm on the surface of the stainless steel pipe of Examples of PatentDocument 1 using this polishing method remains at an area ratio of 3.1%or more. As a result of investigations on the factors of this, it hasbeen found out that the temperature of the surface of the stainlesssteel pipe increases high and an oxide film is formed at the time ofpolishing using a flap wheel, which is a dry polishing and surfacedefects are caused together with polishing marks which are scratchesengraved by high polishing resistance by dry polishing. The term“surface defect” as referred to herein is a defect having a form inwhich the metal on the surface partly peels off and covers the baseportion as the polishing material and polishing paper are continuouslybrought into contact with the surface of the steel pipe and polishedwhen polishing the surface of the steel pipe, and it is referred to as“burr” or “covering”. A surface defect includes portions at which themetal is turned up as a strip shape or a bamboo leaf shape, and it is adefect having a maximum length from one end portion of the portionbonded to the base to the other end portion at the peeling tip of 5 μmor more. The surface defect forms a microgap with the surface baseportion of the stainless steel pipe, and thus crevice corrosion islikely to occur and a decrease in corrosion resistance of the steel pipeis caused.

The present inventors have found out a stainless steel pipe exhibitingexcellent corrosion resistance and a method of manufacturing the samebased on the analysis results.

In other words, the present invention provides a stainless steel pipeexhibiting excellent corrosion resistance and a method of manufacturingthe same of the following (1) to (3).

(1) A stainless steel pipe exhibiting excellent corrosion resistance, inwhich the stainless steel pipe has a polishing mark on a surface, anoxide film exhibiting color is not present on the surface, and anaverage number of surface defects including covering by a metal base of5 μm or more on the surface is suppressed to 5 or fewer per 0.01 mm².

The stainless steel pipe of the present invention exhibits excellentdesign property and antiglare property since it has polishing marks onthe surface thereof. In addition, rusting starting from the oxide filmand the Cr-depleted layer right under the oxide film hardly proceeds andthe corrosion resistance hardly deteriorates since the oxide filmexhibiting color is not present on the surface of the stainless steelpipe. Furthermore, crevice corrosion is suppressed and a stainless steelpipe exhibiting excellent corrosion resistance is obtained since theaverage number of surface defects including covering by the metal baseof 5 μm or more on the surface of the stainless steel pipe is suppressedto 5 or fewer per 0.01 mm².

(2) A method of manufacturing the stainless steel pipe of (1), themethod including a polishing step of polishing a surface of a stainlesssteel pipe with a solid polishing agent.(3) The manufacturing method of (2), in which the surface of thestainless steel pipe is polished by attaching the solid polishing agentto a polishing flap wheel in the polishing step.

Effects of the Invention

According to the present invention, it is possible to provide astainless steel pipe which exhibits excellent corrosion resistance so asnot to rust at an early stage even in a waterfront environment affectedby sea salt particles and a method of manufacturing the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph of the surface enlarged using an opticalmicroscope of a stainless steel pipe, FIG. 1A is a surface on which thegeneration of surface defects is suppressed, and FIG. 1B is a surfacehaving surface defects.

FIG. 2 is a view illustrating the relationship between surface defectsand a change in current density, FIG. 2A is an enlarged photographillustrating surface defects of a stainless steel pipe, and FIG. 2B is agraph illustrating a change in current density in pitting potentialmeasurement.

FIG. 3 is a view illustrating the relationship between surface defectsand a change in current density, FIG. 3A is an enlarged photographillustrating the surface of a stainless steel pipe on which thegeneration of surface defects is suppressed, and FIG. 3B is a graphillustrating a change in current density in pitting potentialmeasurement.

FIG. 4 is a photograph of the surface enlarged using an opticalmicroscope of the stainless steel pipe of Comparative Example 2.

FIG. 5 is a photograph of the surface enlarged using an opticalmicroscope of the stainless steel pipe of Reference Example 1.

FIG. 6 is a photograph of the surface of a stainless steel pipe after aCCT test, FIG. 6A illustrates the surface of Example 1, and FIG. 6Billustrates the surface of Comparative Example 1.

PREFERRED MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments for carrying out the present invention will bedescribed. It should be noted that the present invention is not to beinterpreted restrictively by the embodiments.

(Stainless Steel Pipe)

The stainless steel pipe of the present invention is a stainless steelpipe exhibiting excellent corrosion resistance since it has a polishingmark on the surface, an oxide film exhibiting color is not present onthe surface, and an average number of surface defects including coveringby the metal base of 5 μm or more on the surface is suppressed to 5 orfewer per 0.01 mm².

In the present invention, the surface of the stainless steel pipe issubjected to polishing finish so as to impart unevenness and gloss tothe surface. By this, the stainless steel pipe becomes a stainless steelpipe which have polishing marks and exhibits excellent design propertyand antiglare property. The polishing marks are scratches engraved onthe surface of the stainless steel pipe by polishing.

With regard to the polishing marks on the surface after polishing, thepossibility that the oxide film formed by polishing using a flap wheelor the like remains is higher as the concave portion of the polishingmarks is deeper, and rusting proceeds and corrosion resistance is likelyto deteriorate since the concave portion of the polishing marks becomesa starting point of rusting. Accordingly, the surface roughness Ra ofthe surface of the stainless steel pipe in the present invention afterpolishing is preferably from 0.1 to 1.0 μm and more preferably from 0.2to 0.5 μm. The surface roughness after polishing is measured inconformity to JIS B0601, and it can be measured using a contact typesurface roughness tester, for example.

As polishing finish, dry polishing using a flap wheel or the like hasbeen conventionally conducted, but the temperature of the surface of thestainless steel pipe increases high and an oxide film is formed when drypolishing is conducted. Meanwhile, the stainless steel pipe of thepresent invention is characterized in that an oxide film exhibitingcolor is not present on the surface. The present inventors believe thatthe reason for this is because the oxide film on the surface is removedas the stainless steel pipe of the present invention is polished with asolid polishing agent. In addition, the formation of oxide film isfurther suppressed as the solid polishing agent is attached to thepolishing flap wheel.

In the present invention, the fact that an oxide film exhibiting coloris present refers to a case in which an oxide film which is a spot-likesubstance exhibiting color is present at an area ratio of 5% or more in50 μm square when arbitrary 10 points on the surface of the stainlesssteel pipe are observed at a magnification of 400-fold using an opticalmicroscope. Here, the color is not limited to a specific one, and it maybe a color which can be visually distinguished from the metal base ormetallic luster of the stainless steel pipe. A typical color as thecolor is dark reddish-brown.

In addition, when dry polishing using a flap wheel or the like isconducted as polishing finish, the polishing material and the polishingpaper are continuously brought into contact with the surface of thestainless steel pipe and a surface defect which is burr or covering thatthe metal constituting the surface partly peels off and covers the baseportion is caused. The surface defect is a factor of crevice corrosionsince it forms a microgap with the surface base portion of the stainlesssteel pipe.

FIG. 1 is a photograph of the surface enlarged using an opticalmicroscope of the stainless steel pipe, FIG. 1A is a surface on whichthe generation of surface defects is suppressed, and FIG. 1B is asurface having surface defects. FIG. 1A is the surface of the stainlesssteel pipe of the present invention, which has polishing marks but onwhich the generation of surface defects is suppressed. Meanwhile, FIG.1B is a dry polished surface of a stainless steel pipe, and surroundedportions 1 to 9 represent surface defects that the metal constitutingthe surface partly peels off and covers the base portion. The presentinventors have analyzed the reason why the generation of surface defectsis suppressed on the surface of the stainless steel pipe of the presentinvention after polishing as illustrated in FIG. 1A is because a solidpolishing agent is used at the time of polishing. In addition, thegeneration of surface defects is further suppressed by attaching thesolid polishing agent to the polishing flap wheel. Incidentally, thewhite horizontal line in FIG. 1 represents a convex portion formedduring polishing, and the concave portion between the white horizontalline which is a convex portion and the white horizontal line adjacentthereto is a polishing mark.

In the present invention, the surface defects refer to those havingcovering by the metal base in which the maximum length portion of thedefect has a size of 5 μm or more. In addition, the surface is definedto be in a state in which the generation of surface defects issuppressed in the present invention in a case in which the averagenumber of measured surface defects is 5 or fewer when a range of 100μm×100 μm (0.01 mm²) at arbitrary 10 points on the polished surface of astainless steel pipe is enlarged by 200-fold and observed using anoptical microscope. The number of surface defects on the polishedsurface of the stainless steel pipe is more preferably 3 or fewer andstill more preferably 2 or fewer per unit area of 100 μm×100 μm (0.01mm²). Incidentally, there is no upper limit to the maximum lengthportion of surface defects, but the upper limit may be set to 50 μm as areference value in the measurement.

FIG. 2 and FIG. 3 are views illustrating the relationship betweensurface defects and a change in current density, FIG. 2A is an enlargedphotograph illustrating surface defects of a stainless steel pipe, FIG.3A is an enlarged photograph illustrating the surface of a stainlesssteel pipe on which the generation of surface defects is suppressed, andFIG. 2B and FIG. 3B are graphs illustrating a change in current densityin pitting potential measurement of the stainless steel pipes of FIG. 2Aand FIG. 3A, respectively.

The pitting potential measurement method of stainless steel conforms toJIS G0577, and the method B is used. The method B is a pitting potentialmeasurement method by a potentiodynamic method in a 3.5 mass % sodiumchloride aqueous solution. The pH of the sodium chloride aqueoussolution is set to 7 and the temperature is set to 30° C. In addition,the potential sweep rate is set to 20 mV/min.

As illustrated in FIG. 3A and FIG. 3B, in the case of a stainless steelpipe having a surface on which the generation of surface defects issuppressed, a change in the value of the current density at a potentiallower than the pitting potential is small and the portion at which therate of change (maximum current density/minimum current density) in thecurrent density in a range of from the natural potential to the pittingpotential, namely, a potential range of from 0.1 to 0.5 (part B in FIG.3B) is 10 or more is not recognized in the change in current density inthe pitting potential measurement.

Meanwhile, as illustrated in FIG. 2A and FIG. 2B, in the case of astainless steel pipe having surface defects, the change in the value ofthe current density at a potential lower than the pitting potential isgreat and the portion at which the rate of change in the current densityin a range of from the natural potential to the pitting potential,namely, a potential range of from 0.1 to 0.3 V (part A in FIG. 2B)exceeds 10 is at ten or more potential positions in the change incurrent density in the pitting potential measurement. This great changein current density is caused by occurrence of corrosion. Therefore, thepresent inventors presume that a great change in current densityindicates the occurrence of crevice corrosion caused by the presence ofsurface defects. Accordingly, in the present invention, the portion atwhich the rate of change (maximum current density/minimum currentdensity) in the current density in a range of from the natural potentialto the pitting potential is 10 or more is preferably at fewer than tenpotential positions and more preferably at five or fewer places in thechange in current density in pitting potential measurement.

As a composition in the case of using ferritic stainless steel as thematerial for the stainless steel pipe of the present invention, forexample, C is contained preferably at 0.02% by mass or less since ittends to decrease corrosion resistance when being contained in a greatamount although it is an element useful for obtaining the strength ofsteel. Si is contained preferably at 1.00% by mass or less since ittends to harden the steel when being contained in a great amountalthough it is an element useful as a deoxidizer and a heat source inthe steelmaking process. Mn is contained preferably at 2.00% by mass orless, and more preferably at 1.00% by mass or less since it tends toform an austenite phase when being contained in a great amount althoughit is an element useful as deoxidation in the steelmaking process. Cr iscontained preferably at from 17.00% to 30.00% by mass and morepreferably at from 20.00% to 24.00% by mass since it tends to not onlyincrease the cost but also decrease the processability when beingcontained in a great amount although it is an element useful forsecuring corrosion resistance. Mo is contained preferably at from 1.00%to 20.50% by mass and more preferably at from 1.00% to 1.50% by masssince it tends to not only increase the cost but also decrease theprocessability when being contained in a great amount although it is anelement useful for improving the corrosion resistance of stainless steelin the presence of Cr. It is more preferable as P is contained in asmaller amount since it decreases corrosion resistance, and P iscontained preferably at 0.040% by mass or less. It is more preferable asS is contained in a smaller amount since it decreases corrosionresistance, and S is contained preferably at 0.030% by mass or less. Niis contained preferably at 0.6% by mass or less since it causes theformation of an austenite phase and high cost when being contained toomuch although it is preferred from the viewpoint of an effect ofsuppressing the progress of corrosion and of being effective inimprovement of toughness of ferritic stainless steel pipes. With regardto Ti and Nb, it is preferable to contain one kind or two kinds ofthese. Ti is contained preferably at from 0.05% to 0.5% by mass since alarge amount of Ti content tends to decrease the surface quality ofsteel although it is preferred from the viewpoint of having a strongaffinity for C and N and suppressing intergranular corrosion of ferriticstainless steel pipes. Nb is contained preferably at from 0.1% to 0.6%by mass since a large amount of Nb content tends to hinder toughnessalthough it is preferred from the viewpoint of having a strong affinityfor C and N and suppressing intergranular corrosion of ferriticstainless steel pipes. N is contained preferably at 0.025% by mass orless since it, like C, tends to decrease corrosion resistance when beingcontained in a great amount. Al is contained preferably at from 0.01% to0.50% by mass since it decreases the weldability and low temperaturetoughness of steel as well as deteriorates the surface quality whenbeing excessively added although it is an element effective in refiningand casting as a deoxidizer. The balance is preferably Fe and inevitableimpurities. In addition, for example, it is also possible to use onecontaining C at 0.02% by mass or less, Si at 0.40% by mass or less, Mnat 0.40% by mass or less, Cr at from 21.00% to 23.00% by mass, Mo atfrom 1.00% to 1.50% by mass, P at 0.040% by mass or less, S at 0.030% bymass or less, Ni at 0.60% by mass or less, Ti at from 0.05% to 0.5% bymass, Nb at from 0.10% to 0.6% by mass, N at 0.025% by mass or less, Alat 0.15% by mass or less, and Fe as the balance as the stainless steelpipe of the present invention.

As the material for the stainless steel pipe of the present invention, amaterial having a pitting (corrosion) index (PI) of 20 or more ispreferable. PI is determined by the following Equation (1).

PI=Cr+3Mo  Equation (1)

The stainless steel pipe of the present invention having a pitting index(PI) of 20 or more exhibits excellent corrosion resistance. Hence,rusting of the stainless steel pipe of the present invention can besuppressed whereas SUS304 having a low pitting index of 19 rusts at anearly stage in a waterfront environment affected by sea salt particles.The pitting index (PI) is more preferably 24 or more and still morepreferably 30 or more from the viewpoint of corrosion resistance.

(Manufacturing Method)

The method of manufacturing a stainless steel pipe of the presentinvention is a manufacturing method including a polishing step ofpolishing the surface of a stainless steel pipe with a solid polishingagent.

The solid polishing agent is not particularly limited and can be used aslong as it contains a fatty acid and mineral fat and oil.

It is preferable that the solid polishing agent contains oxides such asSiO₂, Al₂O₃, and CrO₂. The content of oxides such as SiO₂, Al₂O₃, andCrO₂ is preferably from 50% to 80% by mass, more preferably from 55% to75% by mass, and particularly preferably from 60% to 70% by mass.

As the fatty acid, it is preferable to use stearic acid, myristic acid,and the like. As the mineral fat and oil, it is preferable to usepalmitic acid and the like.

In the present method of manufacturing a stainless steel pipe, it ispreferable to polish the surface of the stainless steel pipe using apolishing flap wheel to which the solid polishing agent is attached inthe polishing step.

As described above, when dry polishing using a flap wheel or the like isconducted as polishing finish, the polishing material and the polishingpaper are continuously brought into contact with the surface of thestainless steel pipe and a surface defect which is burr or covering thatthe metal constituting the surface partly peels off and covers the baseportion is caused. Meanwhile, it is preferable to conduct wet polishingby attaching the solid polishing agent to the polishing flap wheel inthe method of manufacturing a stainless steel pipe of the presentinvention. This makes it possible to decrease the polishing resistanceeven in a case in which the polishing material and the polishing paperare continuously brought into contact with the surface of the stainlesssteel pipe, and the generation of a surface defect which is burr orcovering that the metal constituting the surface partly peels off andcovers the base portion is likely to be further suppressed.

Incidentally, the present invention is not limited by the aboveembodiment. For example, buffing using a solid polishing agent may beconducted after wet polishing is conducted by attaching a solidpolishing agent to a polishing flap wheel. In addition, it is possibleto manufacture a stainless steel pipe which has random polishing markson the surface of the stainless steel pipe and on which generation of anoxide film exhibiting color and surface defects is suppressed bymanually conducting polishing by the movement formed by combining aneccentric motion and a rotational motion using a polishing apparatus(air sander) to which a nonwoven fabric is attached after a solidpolishing agent is applied and wet polishing is conducted as well.

EXAMPLES

Piping and shape modification of a stainless steel pipe were conducted,and polishing finish for decoration was conducted. The following twotypes of stainless steel pipes were used. The composition (mass %) anddimensions are as follows.

Steel Type 1 (SUS445J1) Cr: 22%, Mo: 1.05%, Ti: 0.2%, Nb: 0.2%, Al:0.09%, and Fe: balance

Steel Type 2 (SUS304) Cr: 18%, Ni: 8%, Si: 0.6%, Mn: 0.8%, and Fe:balance

Dimensions: 34 mm in diameter×1.5 mm in thickness×4000 mm in length.

Polishing was conducted in Lines 1 to 4 as follows. In addition, thepolishing conditions are as follows.

Line 1 is a line in which five flap wheels (#240, #240, #240, #400, and#600) are arranged so as to polish the surface of steel pipe in thecircumferential direction (to impart polishing marks in thecircumferential direction). Line 2 is a line in which four flap wheels(#240, #240, #240, and #400) are arranged so as to polish the surface ofsteel pipe in the longitudinal direction (to impart polishing marks inthe longitudinal direction). Line 3 is a line in which four flap wheels(#150, #150, #150, and #320) are arranged so as to polish the surface ofsteel pipe in the longitudinal direction (to impart polishing marks inthe longitudinal direction). Line 4 is a line composed of three flapwheels (#320, #400, and #600) arranged so as to polish the surface ofsteel pipe in the longitudinal direction (to impart polishing marks inthe longitudinal direction) and two cotton buffs (#400 and #400)arranged so as to polish the surface of steel pipe in thecircumferential direction (to impart polishing marks in thecircumferential direction). Here, a solid polishing agent was applied tothe flap wheel in Line 1 and Line 4. Meanwhile, a solid polishing agentwas not applied in Line 2 and Line 3. Incidentally, “#240” and the likerepresent the mesh grading.

(Polishing Conditions)

Line speed: 1.8 m/min

Number of rotations of pipe: 380 rpm

Number of rotations of wheel: 1500 rpm

Wheel diameter: 400 mm

(Solid Polishing Agent)

The solid polishing agent had a SiO₂ content of 75% by mass, a contentof stearic acid which was a fatty acid of 16% by mass, and a content ofpalmitic acid which was a mineral fat and oil of 3.8% by mass.

Example 1

Polishing of Steel Type 1 was conducted in Line 1 (applied with a solidpolishing agent).

Example 2

Polishing of Steel Type 1 was conducted in Line 3 (not applied with asolid polishing agent) and then in Line 4 (applied with a solidpolishing agent). Thereafter, polishing to uniformly adjust the randompolishing marks was manually conducted by the movement formed bycombining an eccentric motion and a rotational motion using a polishingapparatus (air sander) to which a nonwoven fabric (#80) was attachedwithout applying a solid polishing agent.

Comparative Example 1

Polishing of Steel Type 1 was conducted in Line 2 (not applied with asolid polishing agent).

Comparative Example 2

Polishing of Steel Type 2 was conducted in Line 2 (not applied with asolid polishing agent).

Reference Example 1

Polishing of Steel Type 2 was conducted in Line 1 (applied with a solidpolishing agent).

(Surface Defects)

The polished surface of a stainless steel pipe was enlarged by 200-foldand observed in a range of 100 μm×100 μm (0.01 mm²) using an opticalmicroscope. The surface was evaluated as “◯” to be in a state in whichthe generation of surface defects was suppressed in a case in which thenumber of surface defects having covering by the metal base of 5 μm ormore was 5 or fewer, and the surface was evaluated as “x” to be in astate in which the generation of surface defects was suppressed in acase in which the number of surface defects was more than 5 (see Table1).

As presented in Table 1, the surface of the stainless steel pipe ofExample 1 did not have surface defects as illustrated in FIG. 1A.Meanwhile, the surface of the stainless steel pipe of ComparativeExample 1 had at least nine surface defects as illustrated in FIG. 1B,and thus it was not in a state in which the generation of surfacedefects was suppressed. In addition, the surface of the stainless steelpipe of Comparative Example 2 had at least 6 surface defects asillustrated in FIG. 4, and thus it was not in a state in which thegeneration of surface defects was suppressed. Incidentally, in ReferenceExample 1, there was no surface defect as illustrated in FIG. 5.

(Oxide Film)

The surface of the stainless steel pipe was observed at a magnificationof 400-fold using an optical microscope, and to what extent an oxidefilm which was a spot-like substance exhibiting dark reddish-brown waspresent in 50 μm square as an area ratio was calculated. The surface wasevaluated as “◯ (Good)” not to have an oxide film exhibiting color in acase in which the area ratio of the remaining oxide film was 3% or moreand less than 5%, the surface was evaluated as “⊙ (Excellent)” in thecase of a more preferred state in which the area ratio of the remainingoxide film was less than 3%, and the surface was evaluated as “x(Failure)” to have an oxide film exhibiting color in a case in which thearea ratio was 5% or more (see Table 1).

As presented in Table 1, the area ratio of the oxide film was 1% or lessin Example 1, the area ratio of the oxide film was 3% in Example 2, andan oxide film exhibiting color was not thus present on the surface ofthe stainless steel pipes. Meanwhile, the area ratio of the oxide filmwas 15% and 20% in Comparative Examples 1 and 2, respectively and anoxide film exhibiting color was thus present on the surface of thestainless steel pipes. Incidentally, the area ratio of the oxide filmwas 2% and an oxide film exhibiting color was not thus present on thesurface of the stainless steel pipe in Reference Example 1.

(Corrosion Resistance Test)

The stainless steel pipes of Examples 1 and 2, Comparative Examples 1and 2, and Reference Example 1 were subjected to a corrosion resistancetest (salt-dry-wet combined cyclic corrosion test (CCT test)) under thefollowing conditions. Conditions: (1) Salt water spray (35° C., 5% NaCl,15 minutes)

(2) Drying (60° C., 30% RH, 60 minutes)(3) Wetting (50° C., 95% RH, 3 hours)The above conditions (1) to (3) constituted one cycle, and the cycle wasrepeatedly conducted by 30 cycles. Evaluation: the stainless steel pipewas evaluated as “◯ (Good)” to exhibit good corrosion resistance whenthe rusting area after the test was within 5% of the entire surface ofthe steel pipe, as “Δ (Passing)” when the rusting area was more than 5%and 15% or less, and as “x (Failure)” to exhibit poor corrosionresistance when the rusting area was more than 15% (see Table 1).

The surface photographs after the CCT test of Example 1 and ComparativeExample 1 are illustrated in FIG. 6. In Example 1, it has been indicatedthat rusting has not occurred on the surface even after the CCT test asillustrated in FIG. 6A and the corrosion resistance is excellent.Meanwhile, in Comparative Example 1, rusting has occurred on the surfaceafter the CCT test as illustrated in FIG. 6B and the corrosionresistance is inferior. Incidentally, in Reference Example 1, thecorrosion resistance is Δ since the corrosion resistant level of thebase material itself is low. The corrosion resistant level of the basematerial in a waterfront environment affected by sea salt particles ispreferably 24 or more as a pitting index (PI).

TABLE 1 Corrosion Surface defects Oxide film resistance Example1 ◯ ⊙ ◯Example2 ◯ ◯ ◯ Comparative Example1 X X X Comparative Example2 X X XReference Example1 ◯ ⊙ Δ

EXPLANATION OF REFERENCE NUMERALS

-   -   1 to 9 . . . Surface defect    -   A and B . . . Region having change in current density

1. A stainless steel pipe exhibiting excellent corrosion resistance,wherein the stainless steel pipe has a polishing mark on a surface, anoxide film exhibiting color is not present on the surface, and anaverage number of surface defects including covering by a metal base of5 μm or more on the surface is suppressed to 5 or fewer per 0.01 mm². 2.A method of manufacturing the stainless steel pipe according to claim 1,the method comprising a polishing step of polishing a surface of astainless steel pipe with a solid polishing agent.
 3. The manufacturingmethod according to claim 2, wherein the surface of the stainless steelpipe is polished by attaching the solid polishing agent to a polishingflap wheel in the polishing step.